1. Field of the Invention
This invention relates to a semiconductor laser device whose output beam is made stable, and also to a semiconductor laser device the beam intensity of whose output beam can be detected. The invention further relates to a semiconductor laser device which is suited for use in an electrophotographic image recording apparatus using a photosensitive medium. The invention further relates to an image recording apparatus using the output beam of a semiconductor laser device.
2. Description of the Prior Art
There has heretofore been proposed a semiconductor laser device adopting the so-called auto power control (hereinafter referred to as APC) in which the beam output of a semiconductor laser element is detected and fed back and controlled to thereby maintain the light output intensity of the semiconductor laser element constant.
As an example of the device having such APC function, there has been proposed a semiconductor laser device in which the principal beam (hereinafter referred to as the front beam) of a semiconductor laser element which provides the output of a light source device and the subordinate beam thereof (hereinafter referred to as the rear beam) become identical with respect to power and emission distribution and therefore the rear beam is fed back to a laser driving circuit to thereby control the front beam.
For example, in the device disclosed in U.S. Pat. No. 4,125,777, as shown in FIG. 1 of the accompanying drawings, a light-receiving element 3 for receiving the rear beam 2 of a semiconductor laser element 1 to effect APC is provided and the signal thereof is sent to a separate laser driving circuit so as to control the output of the front beam 4 which provides the original output of a light source device.
In these devices, however, the rear beam emitting surface 5 of the semiconductor laser element 1 is opposed to the light-receiving surface 6 of the light-receiving element 3 and this leads to the following problem.
That is, if the emergence angle of the rear beam 2 with respect to the perpendicular 7 to the rear beam emitting surface 5 of the semiconductor laser element 1 is .alpha. and the intensity thereof is I(.alpha.)(W/sr), the distribution of I(.alpha.) becomes such as indicated by solid line A in FIG. 2 of the accompanying drawings and is similar to gaussian distribution. This entirely holds true with the front beam.
Next, let l(m) be the distance between the rear beam emitting surface 5 and the light-receiving surface 6 and L(.alpha.) be the power (illumination) per unit area of the light-receiving surface 6 receiving the rear beam having an emergence angle .alpha.. Then, EQU L(.alpha.)=I(.alpha.)/l.sup.2 cos.sup.3 .alpha. (1)
The relation of equation (1) is such as indicated by dotted line B in FIG. 2, from which it is seen that the illumination L(O) of the central portion of the light-receiving surface receiving the peak beam .alpha.=0 becomes maximum, and L(O) is EQU L(O)=I(O)/l.sup.2 ( 2)
As the marginal portion is approached, the illumination of cos.sup.3 .alpha. is reduced in addition to the reduction in I(.alpha.). Accordingly, the light-receiving surface 6 in the device of the aforementioned U.S. patent has a great difference in illumination between the central portion and marginal portion thereof, and this leads to the following problem.
Generally, if the illumination of the light-receiving surface is L(W/m.sup.2) and the terminal voltage (output voltage) is E(V), the output characteristic of the light-receiving element does not always become linear, but is divided into a straight range G and non-straight ranges F and H, as shown in FIG. 3 of the accompanying drawings.
In the straight range G, the terminal voltage E increases in proportion to an increase in illumination L. However, in the non-straight ranges F and H, the terminal voltage E is not proportional to an increase in illumination L and particularly, in the range F, the S/N ratio of the light-receiving element is reduced.
Accordingly, if the illumination difference between the central portion and marginal portion of the light-receiving surface 6 is great as in the device of the aforementioned U.S. patent, the illumination in the central portion becomes the range H of FIG. 3 and conversely, the illumination in the marginal portion becomes the range F. As a result, the output voltage of the light-receiving element 3 is not proportional to the illumination and it may become impossible to effect highly accurate detection of the quantity of light.
In addition to such a problem, the device as shown in FIG. 1 encounters a problem that the rear beam 2 is reflected to the light-receiving surface of the light-receiving element 3 and mixes and interferes with the front beam 4 and moreover, when the front beam is imaged, the ghost of the rear beam reflected by the light-receiving surface appears.
Heretofore, in the field of light communications and the like, semiconductor laser elements have been widely used as the light source. In such cases, the aforementioned APC is often adopted.
FIG. 4 of the accompanying drawings illustrates such method. The light output of a semiconductor laser element 11 is received by a photodetector 12 and amplified by an amplifier 13, whereafter it is fed back to a laser driving circuit 14 and controlled so that the output of the photodetector 12 is always constant. However, where the output light from the semiconductor laser element is used for the image recording or the like, the wavelength dependency of the sensitivity of a photosensitive medium irradiated with the laser light raises a great problem. In a device using a laser such as conventional He-Ne laser having a short wavelength, the spectrum sensitivity of the photosensitive medium near the laser wavelength is often flat, thus permitting the conventional APC system to be adopted. On the other hand, the semiconductor laser emitted from the semiconductor laser element is of a wavelength of about 8000 A which is in the near-infrared range as compared with the He-Ne laser and therefore, the photosensitive medium usually used is low in sensitivity. Therefore, where a semiconductor laser is used in such an image recording apparatus, it is often the case that the photosensitive medium is sensitized for use. However, even if it is sensitized, the spectrum sensitivity thereof cannot be made flat even in the semiconductor laser wavelength range in terms of the stability of the image quality and the durability of the photosensitive medium, and still has dependency on the wavelength as shown in FIG. 5 of the accompanying drawings.
It is therefore necessary to control the quantity of light depending on the wavelength of the semiconductor laser element used and if a wavelength fluctuation occurs during the operation of the semiconductor laser element, no good image can be obtained by the conventional APC system.
Describing an example, a semiconductor laser has a temperature coefficient of wavelength of 2.5-3.0 A and if there is a temperature variation of 30.degree. C., it creates a wavelength variation of maximum 90 A. Thus, as the wavelength dependency of the sensitivity of the photosensitive medium is higher, the image obtained becomes worse in quality.
On the other hand, there is a method of using a semiconductor laser element at a predetermined temperature with the aid of cooling means such as Peltier element, but this method employs a number of components and is high in cost.